A pneumatic brake booster is used in a braking system to pneumatically amplify the muscle force applied to a brake pedal. The booster is arranged between the brake pedal and a master cylinder, for example a tandem master cylinder.
It comprises a casing divided into two chambers by a sealed moving wall, this wall being able to move by a pressure differential between the two chambers. The moving wall comprising a pneumatic piston extending radially outwards via a skirt crimped to the external periphery of the piston. The pressure differential between the two chambers is controlled by a three-way valve subject to the movement of an actuating rod controlled by a brake pedal. This three-way valve is arranged in the rear cylindrical part of the pneumatic piston.
The action of the actuating rod is transmitted to the pistons of a master cylinder by means of a push rod via a reaction disc made of an elastomeric material, for example rubber. The reaction disc is a means of combining three forces, a first force from the actuating rod applied via a plunger distributor which is fixed with respect to the actuating rod, the second force from the boost supplied by the pneumatic piston as a result of the movement of the moving wall by the pressure differential, and a third force being the reaction of the hydraulic braking circuit which is transmitted through the push rod. This disc allows the driver of the vehicle to adjust his action on the brake pedal by feeding information back to him from the hydraulic circuit.
The force applied by the driver to the pedal will be known as the input force; the force applied by the actuating rod will be known as the actuating force and the force exerted by the push rod on a piston of a master cylinder, for example a tandem master cylinder, will be known as the output force.
By virtue of this type of brake booster it is possible to obtain a fairly refined relationship between the input force and the output force over a certain range of input force values.
There are devices which allow the boost of the booster to be varied, that is to say which allow the ratio between the input force and the output force to be varied, for example so as to increase the boost above and beyond a certain input force. Patent FR 00/15943 describes a device applied to the rear cylindrical part of the pneumatic piston, the distributor is made up of two parts, a plunger on the same side as the actuating rod and a feeler on the same side as the reaction disc, a prestress spring in a deformable cage is applied between the feeler and the plunger. For an actuating force below the spring prestress, the feeler and the plunger behave like a rigid piece. For an input force that is high enough, the spring compresses, the feeler can then move axially towards the brake pedal allowing the reaction disc to expand towards the brake pedal. There is therefore a reduction in the reaction force transmitted to the actuating rod, and this increases the boost ratio.
However, this device entails significant modification to the internal structure of the booster because of its bulk, although for cost reasons equipment manufacturers are seeking to standardize their products. In addition, they are also seeking to reduce the size of the boosters.